Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, such as described in U.S. Pat. Nos. 3,455,510 and 3,171,599 (both Rotolico, now assigned to the present assignee), a low velocity combustion flame is used and the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. The carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. Other heating means may be used as well, such as arc plasmas, electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters.
The material alternatively may be fed into a heating zone in the form of a rod or wire such as described in U.S. Pat. Nos. 3,148,818 (Charlop) and 2,361,420 (Shepard). In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized by an atomizing blast gas such as compressed air, and thence propelled in finely divided form onto the surface to be coated.
A newer, rocket type of spray gun is typified in U.S. Pat. No. 4,416,421 (Browning). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed through an annular opening into the constricted throat of a long nozzle chamber. Powder or wire is fed axially within the annular opening into the nozzle chamber to be heated and propelled by the combustion effluent.
Short-nozzle spray devices are disclosed for high velocity combustion spraying in French Patent No. 1,041,056 (Union Carbide Corp.) and U.S. Pat. No. 2,317,173 (Bleakley). Powder is fed axially into a melting chamber within an annular flow of combustion gas. An annular air flow is injected coaxially outside of the combustion gas flow, along the wall of the chamber. The spray stream with the heated powder issues from the open end of the combustion chamber.
These short-nozzle devices have a nozzle construction similar to commercial wire spray guns of the type disclosed in the aforementioned U.S. Pat. No. 3,148,818. However, wire guns function quite differently, the combustion flame melting the wire tip which extends about 0.5 to 1.0 inches from the air cap on the gun, and the air atomizing the molten material from the tip and propelling the droplets. Wire guns generally have been used to spray only at moderate velocity, again despite having been in widespread commercial use for over 50 years.
Thermal spray guns generally are directed to spraying either powder or wire, rather than spraying both simultaneously. An exception is U.S. Pat. No. 3,312,566 (Winzeler et al; FIG. 6 thereof) which discloses a plasma spray gun in which a rod is fed into one side of the plasma jet, and powder is fed into the other side. Those skilled in the art will recognize a tendency for feed material to ride the side of the plasma jet whence the material is fed. Therefore, less than complete commingling of the rod material and powder material may be expected in the spray stream.
Another exception is U.S. Pat. No. 2,233,304 (Bleakley) which discloses an attachment to a combustion wire (rod) gun for introducing powder such as graphite forward and annularly outward of the heating flame and atomizing gas. Although directed to mixing the powder and wire material in the coating, the patent expressly provides for separation of the powder from the adjacent molten particles by the atomizing gas.
Composite wire formed of an alloy sheath and a powder core is described in U.S. Pat. No. 4,741,974 (Longo et al) of the present assignee. Such wire has been quite successful for thermal spraying, but requires special manufacture and does not allow full choice of materials and relative proportions of the sheath alloy and core materials.
Since thermal spraying involves melting or at least surface heat softening the spray material, difficult-to-melt powders such as most carbides, borides and nitrides cannot be fed into the gun without incorporating a binder into the material. Thus a material such as tungsten carbide powder typically has an integral cobalt binder fused or sintered with the carbide. Other powders for thermal spraying are formed by compositing or cladding one material onto a core of another material. Such requirements add to costs and limit versatility of coating compositions. Also, the compositing or cladding has not been fully sufficient for producing the most desirable quality coatings and optimum deposit efficiency with ordinary thermal spray guns.
Therefore objects of the present invention are to provide an improved thermal spray apparatus for simultaneous spraying of wire and powder, to provide a thermal spray gun for wire and powder in which the wire material and the powder have improved commingling in the spray stream, to provide a novel thermal spray gun in which wire and powder are fed independently, to provide thermal spray apparatus and method for producing novel coatings, to provide a method and apparatus for producing dense tenacious thermal sprayed coatings, and to provide a novel method and apparatus for combustion thermal spraying at high velocity.